Well, its no surprise that new Oasis tracks have leaked online...the catch is that these tracks have been floating around for some time in the fan community.
The tracks that have "leaked" are "Stop The Clocks", "Nothin' On Me", and "I Wanna Live My Dream (In A Record Machine)" and are from 2005's 'Don't Believe The Truth" demos. Regardless, now they will be up for grabs around the internet and you should give them a listen. In fact, give them a lot of listens. Its Oasis...they never do anything bad.
Here they are courtesy of YouTube (sorry for the lack of MP3s!):
"Stop The Clocks":
"I Wanna Live My Dream (In A Record Machine)":
"Nothin' On Me":